STM and TVar report incorrect results

On Haskell Cafe, I posted:

I'm getting strange behavior when using the 'many' combinator to read zero or more items off of a TQueue with readTQueue. The script that exhibits this behavior is as follows:

import Control.Concurrent.STM
import Control.Concurrent
import Control.Monad
import Control.Applicative

main = do
    q <- newTQueueIO
    atomically $ writeTQueue q True
    atomically $ writeTQueue q False
    forever $ do
        xs <- atomically $ many $ readTQueue q
        print xs
        threadDelay 500000

I'd expect the output of the script to be:

[True,False]
[]
[]
...

However, that is not the case: the actual output of the script is:

[True,False]
[True,False]
[True,False]
...

If 1 element (say, True) is written into the TQueue instead of 2, then the output of the script is:

[True]
[]
[]
...

Which is expected behavior, but inconsistent with the behavior when the TQueue has 2 or more elements in it.

Is this considered a bug, or undocumented behavior of TQueue?

---

Bas vas Dijk noted that this may be a bug in STM, and provided a condensed test case which reproduces the behavior of my original script:

$ cat stmTest.hs
import Control.Concurrent.STM
main = do
  x <- atomically $ do
         t <- newTVar 1
         writeTVar t 2
         ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
         readTVar t
  print x

$ ghc --make stmTest.hs -fforce-recomp -threaded -o stmTest && ./stmTest
[1 of 1] Compiling Main             ( stmTest.hs, stmTest.o )
Linking stmTest ...
1

The program prints 1 when it should print 2.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	v.dijk.bas@gmail.com
Operating system
Architecture